The primary objective of this proposal is to understand the molecular processes responsible for tension generation and contraction in mammalian muscle. It is now well established that the tension generating event involves a change in an ATP-induced conformation of the myosin cross-bridge when it attaches to the actin filament. We shall be primarily concerned with elucidating the structural basis for the conformational changes which myosin undergoes when it interacts with MgATP and actin. We shall approach this by examining how the two essential sulfhydryl groups SH1 and SH2 are perturbed by these interactions. Since we have already demonstrated that these two groups have a close spatial proximity we intend to use a series of site specific, bifunctional reagents of differing crosslinking span to characterize the conformation of myosin in terms of the reactivities of these thiol groups and more importantly in terms of their proximity to one another. Additionally, we shall try to adapt a new procedure for cyanylation cleavage at cysteinyl peptides to myosin, to examine the sulfhydryl role in the ATPase mechanism of myosin and actomysin.